Waveguide-based touch system employing interference effects

ABSTRACT

A touch system that employs interference effects is disclosed. The touch system includes first and second waveguides that have first and second optical paths that define an optical path difference. The first and second waveguides are configured so that a touch event deforms at least one of the waveguides in a manner that causes the optical path difference to change. The change in the optical path difference is detected by combining the light traveling in the two waveguides to form interfered light. The interfered light is processed to determine the occurrence of a touch event. The time-evolution of the deformation at the touch-event location can also be determined by measuring the interfered light over the duration of the touch event.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/651,136 filed on May 24, 2012,the content of which is relied upon and incorporated herein by referencein its entirety.

FIELD

The present disclosure relates to touch-sensitive devices, and inparticular to touch systems that employ interference effects.

BACKGROUND ART

The market for displays and other devices (e.g., keyboards) havingnon-mechanical touch functionality is rapidly growing. As a result,touch-sensing techniques have been developed to enable displays andother devices to have touch functionality. Touch-sensing functionalityis gaining wider use in mobile device applications, such as smartphones, e-book readers, laptop computers and tablet computers.

Touch systems in the form of touch screens have been developed thatrespond to a variety of types of touches, such as single touches,multiple touches, swiping, and touches. Some of these systems rely onlight-scattering and/or light attenuation. While effective, thereremains a need for alternative optics-based approaches to touch-sensingthat can provide the required sensitivity to sense one or more touchevents and to determine the locations of the one or more touch events.

SUMMARY

A touch system that employs optical interference effects is disclosed.The touch system includes first and second waveguides that have firstand second optical paths that define an optical path difference betweena light source and a detector. The first and second waveguides areconfigured so that a touch event deforms at least one of the waveguidesin a manner that causes the optical path difference to change. Thechange in the optical path difference (“optical path change”) isdetected by combining the light traveling in the two waveguides to forminterfered light. The interfered light is processed to determine theoccurrence of a touch event. The time-evolution of the deformation of atleast one of the waveguides at the touch-event location can also bedetermined by measuring the interfered light over the duration of thetouch event.

Aspects of the disclosure include forming an array of opticalinterferometers that can sense a touch event by being sensitive tooptical path changes caused by deforming at least one arm of theinterferometer. The interferometers are waveguide based and are definedby a waveguide assembly that includes first and second waveguides thatdefine first and second interferometer arms.

A network of the waveguide-based interferometers can be used to providetouch-sensing capability over an area of the touch-screen system.Optical fibers, and in particular dual-core optical fibers or like fiberconfigurations, can be used to define the interferometer arms. Theinterferometers can be configured as Mach-Zehnder interferometers and inparticular unbalanced Mach-Zehnder interferometers. The waveguides canhave graded-index or step-index profiles that either support a singleguided mode or multiple guided modes. In an example, only thelowest-order guided mode is used to define the optical path for thegiven waveguide. The waveguides can be supported by a support substrateor can be separated by an air gap. The support substrate can betransparent or opaque, and can be somewhat flexible or pliable tofacilitate localized bending of one or both waveguides.

In examples, only one of the waveguide deforms when the waveguideassembly is subjected to a touch event. In other examples, bothwaveguides deform.

In an example, a detection grid is formed using the waveguides to enable(x,y) detection of the touch event location. This has particular utilityfor touch-sensitive systems such as keyboards that operate based onpressure being applied to a select location on a touch screen. The gridcan be formed in one example using crossed arrays of dual-core opticalfibers. Such a grid can constitute a network of optical interferometers,which each interferometer generating an interferometer signal. Theinterferometer signals can be processed by a controller to establish thelocation as well as the amount of force (e.g., the relative amount offorce or pressure) applied at a touch event locations.

Additional features and advantages of the disclosure are set forth inthe detailed description that follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the disclosure as described herein, including the detaileddescription that follows, the claims, and the appended drawings.

The claims as well as the Abstract are incorporated into and constitutepart of the Detailed Description set forth below.

All publications, articles, patents, published patent applications andthe like cited herein are incorporated by reference herein in theirentirety, including U.S. Patent Application Publication No. 2011/0122091and U.S Provisional Patent Applications Nos. 61/564,003, 61/564,024 and61/640,605.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a face-on view of an example touch system according to thedisclosure;

FIGS. 2A and 2B are top-down views of an example light source thatincludes multiple light-source elements (FIG. 2A) and an exampledetector that includes multiple detector elements (FIG. 2B);

FIG. 3 is a cross-sectional view of an example embodiment of alight-guiding assembly for the touch system of FIG. 1;

FIGS. 4A and 4B are cross-sectional views of two different exampleembodiments for the light-guiding assembly showing the relevantparameters that describe a relative change in the optical paths for thetop and bottom waveguides, wherein in FIG. 4A only the top waveguideflexes when pressure is applied, and wherein in FIG. 4B both the top andbottom waveguides flex when pressure is applied;

FIG. 5A is an elevated view of an example light-guiding assembly whereinthe top and bottom waveguides have a different modulus of elasticity sothat the flexing of the assembly is more localized at the touch locationof a touch event;

FIG. 5B is similar to FIG. 5A and illustrates an embodiment wherein thelight-guiding assembly includes a support structure with a differentmodulus of elasticity than the top and bottom waveguides;

FIGS. 5C and 5D illustrate an example embodiment wherein the curvatureof the top waveguide (FIG. 5C) or both the top and bottom waveguides(FIG. 5D) associated with a touch event is relatively localized;

FIG. 6A is a cross-sectional view of an example embodiment of thelight-guiding assembly wherein the top and bottom waveguides have agradient-index formed, for example, by an ion-exchange process;

FIG. 6B is similar to FIG. 6A and illustrates an example embodimentwherein the top and bottom waveguides are formed as layered structureson the top and bottom of a glass substrate;

FIG. 6C is similar to FIG. 6B and illustrates an example embodimentwherein the support substrate is opaque and the top and bottomwaveguides include an additional cladding layer adjacent the supportsubstrate;

FIG. 7A through FIG. 7C illustrate example light-guiding assemblies,with FIG. 7A showing the assembly prior to a touch event, and FIGS. 7Band 7C showing a touch event occurring at two different touch locations;

FIG. 8A through 8C illustrate three example embodiments for thelight-guiding assembly;

FIGS. 9A and 9B are plots of the operating wavelength λ (μm) vs.(normalized) detected power as a function of the change in the opticalpaths ΔOPD₂ for the top and bottom waveguides as can occur due to atouch event, wherein for FIG. 9A, ΔOPD₂=7.5 μm and for FIG. 9B, ΔOPD₂=15μm;

FIG. 10A is a top-down view of an example light-guiding assembly showinglight traveling over the different optical paths of the top and bottomwaveguides;

FIG. 10B is a top-down view of an example pressure-sensing touch-screensystem in the form of a keyboard, and illustrating regions of surfaceroughness placed in select locations, such as where the individual keyson the keyboard reside;

FIG. 11A is a cross-sectional view of an example dual-core opticalfiber;

FIG. 11B is a cross-sectional view of an example two-fiber assembly thatis analogous to the dual-core optical fiber of FIG. 11A;

FIG. 12 is a schematic diagram of an example optical-fiber-basedMach-Zehnder interferometer suitable for forming a fiber-basedlight-guiding assembly, wherein fibers F1 and F2 correspond to top andbottom waveguides;

FIG. 13A is an elevated view of an example light-guiding assembly thatincludes top and bottom arrays of dual-core optical fibers that form adetection grid that enables (x,y) detection of a touch event;

FIG. 13B is a close-up cross-sectional view of an example light-guidingassembly of FIG. 12;

FIGS. 14A through 14C are similar to FIGS. 7A through 7C and illustratehow the flexing of an optical fiber can give rise to an optical pathdifference between the two cores of the optical fiber and thereforegenerate interfered light that can be detected at the detector;

FIG. 15A is an elevated view of an example display system formed byoperably arranging the touch system disclosed herein adjacent and above(e.g., atop) a conventional display unit; and

FIG. 15B is a more detailed cross-sectional view of the example displaysystem formed by combining the touch system with the conventionaldisplay.

Cartesian coordinates are shown in certain of the Figures for the sakeof reference and are not intended as limiting with respect to directionor orientation.

DETAILED DESCRIPTION

The present disclosure can be understood more readily by reference tothe following detailed description, drawings, examples, and claims, andtheir previous and following description. However, before the presentcompositions, articles, devices, and methods are disclosed anddescribed, it is to be understood that this disclosure is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified, as such can, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

The following description of the disclosure is provided as an enablingteaching of the disclosure in its currently known embodiments. To thisend, those skilled in the relevant art will recognize and appreciatethat many changes can be made to the various aspects of the disclosuredescribed herein, while still obtaining the beneficial results of thepresent disclosure. It will also be apparent that some of the desiredbenefits of the present disclosure can be obtained by selecting some ofthe features of the present disclosure without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present disclosure are possible andcan even be desirable in certain circumstances and are a part of thepresent disclosure. Thus, the following description is provided asillustrative of the principles of the present disclosure and not inlimitation thereof.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are embodiments of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein.

Thus, if a class of substituents A, B, and C are disclosed as well as aclass of substituents D, E, and F, and an example of a combinationembodiment, A-D is disclosed, then each is individually and collectivelycontemplated. Thus, in this example, each of the combinations A-E, A-F,B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated andshould be considered disclosed from disclosure of A, B, and/or C; D, E,and/or F; and the example combination A-D. Likewise, any subset orcombination of these is also specifically contemplated and disclosed.Thus, for example, the sub-group of A-E, B-F, and C-E are specificallycontemplated and should be considered disclosed from disclosure of A, B,and/or C; D, E, and/or F; and the example combination A-D. This conceptapplies to all aspects of this disclosure including, but not limited toany components of the compositions and steps in methods of making andusing the disclosed compositions. Thus, if there are a variety ofadditional steps that can be performed it is understood that each ofthese additional steps can be performed with any specific embodiment orcombination of embodiments of the disclosed methods, and that each suchcombination is specifically contemplated and should be considereddisclosed.

Touch System

FIG. 1 is a schematic diagram of an example touch system 10 (alsoreferred to as a “touch-screen system”) according to the disclosure. Thetouch system 10 may be used in a variety of consumer electronicarticles, for example, in conjunction with displays for cell-phones,keyboards, touch screens and other electronic devices such as thosecapable of wireless communication, music players, notebook computers,mobile devices, game controllers, computer “mice,” electronic bookreaders and the like.

The example touch system 10 of FIG. 1 includes a generally planarlight-guiding assembly (“assembly”) 20, the details of which aredescribed in greater detail below. At least one light source 100 and atleast one detector 200 are disposed adjacent a perimeter P of theassembly and are optically coupled through waveguides of the assembly asexplained in greater detail below. One light source 100 and one detector200 are shown by way of example. Perimeter P includes an edge (end) 23adjacent light source 100 and an edge (end) 24 adjacent detector 200.Perimeter P can have any reasonable shape and is shown as beingrectangular by way of example. Assembly 20 can be rectangular and in theexample shown has a dimension (length) LX in the X-direction and alength LY in the Y-direction.

Multiple light-sources 100 can be used (or equivalently, a light sourcewith multiple light-source elements can be used), and multiple detectors200 can be used (or equivalently, a detector with multiple detectorelements, especially when the location of one or more touch events needsto be determined. In addition, one or more light sources 100 and one ormore detectors 200 can be operably disposed in the assembly to ensurethat the entire (or substantially the entire) assembly can be used tosense the pressure of a touch event. This may include, for example,cycling the activation of sets (e.g., pairs) of light sources 100 and/ordetectors 200 to ensure that all possible locations for touch events arecovered. In an example, the cycling can be done at a rate that is muchfaster than the typical duration of a touch event that applies pressureto elicit a response from touch system 10.

Example detectors 200 include photodiodes and the various types ofphotosensors. Example light sources 100 include LEDs, laser diodes,optical-fiber-based lasers, extended light sources, and the like.

With reference to FIG. 2A, light source 100 can comprise one or morelight-source elements 102 that are operably mounted on flex-circuitboards (“flex circuits”) 110, which in turn are mounted to printedcircuit boards (PCB) 112 arranged adjacent an edge 26 of transparentsheet 20. In the discussion herein, light source 100 can thus mean alight source having one or more light-source elements 102. Likewise,with reference to FIG. 2B, detector 200 can include a detector that hasone more detector elements 202. Interfered light 104AB is shown incidentupon one of detector elements 202.

In example embodiments of the disclosure, an amount of pressure (e.g., arelative amount of pressure) is applied to planar light-guiding assemblyat touch location TL associated with a touch event TE. Aspects of thedisclosure are directed to sensing the occurrence of a touch event TE,while other aspects include the additional function of determining thetouch location TL of the touch event. Other aspects of the disclosureinclude sensing the occurrence of multiple touch events TE andoptionally determining the touch locations TE of the touch events. Otheraspects of the disclosure include sensing both the amount of pressureapplied and determining the touch location TL of a touch event TE.

In an example, touch system 10 includes an optional cover 40 that servesto cover light source 100 and 200 so that they cannot be seen from aboveassembly 20 by a viewer (see, e.g., viewer 500, FIG. 15B). In anexample, cover 40 serves the role of a bezel. In an example, cover 40can be any type of light-blocking member, film, paint, glass, component,material, texture, structure, etc. that serves to block at least visiblelight and that is configured to keep some portion of touch system 10from being viewed by a user, or that blocks one wavelength of lightwhile transmitting another.

In example embodiments, cover 40 can reside anywhere relative toassembly 20 that serves to block a viewer from seeing light source 100or detector 200. Cover 40 need not be contiguous and can be made ofsections or segments. Further, cover 40 can be used to shield detector200 from receiving light other than light 104 from light source 100,such as for sunlight rejection. Thus, in an example, cover can besubstantially opaque at one wavelength (e.g., a visible wavelength) andsubstantially transparent at another wavelength (e.g., an infraredwavelength for light 104 from light source 100).

In an example, cover 40 is in the form of a film that is opaque at leastat visible wavelengths and that optionally transmits at IR wavelengths.An example film for cover 40 comprises a black paint that absorbs lightover a wide range of wavelengths including the visible and IRwavelengths.

With continuing reference to FIG. 1, touch system 10 may include acontroller 300 that is operably connected (e.g., via a bus 301) to theone or more light sources 100 and the one or more detectors 200.Controller 300 is configured to control the operation of touch system10. In some embodiments, the controller 300 includes a processor 302, adevice driver 304 and interface circuit 306, all operably arranged.Controller controls light source 100 via a light-source signal SL andalso receives and processes a detector signal SD from detector 200.

In an example, controller 300 comprises a computer and includes adevice, for example, a floppy disk drive, CD-ROM drive, DVD drive,magnetic optical disk (MOD) device (not shown), or any other digitaldevice including a network connecting device such as an Ethernet device(not shown) for reading instructions and/or data from acomputer-readable medium, such as a floppy disk, a CD-ROM, a DVD, a MODor another digital source such as a network or the Internet, as well asyet to be developed digital means. The computer executes instructionsstored in firmware and/or software (not shown).

The computer is programmable to perform functions described herein,including the operation of the touch system and any signal processingthat is required to measure, for example, relative amounts of pressure,as well as the location of a touch event, or multiple touch events andmultiple pressures. As used herein, the term computer is not limited tojust those integrated circuits referred to in the art as computers, butbroadly refers to computers, processors, microcontrollers,microcomputers, programmable logic controllers, application-specificintegrated circuits, and other programmable circuits, and these termsare used interchangeably herein.

Software may implement or aid in performing the pressure-sensingfunctions and operations disclosed herein. The software may be operablyinstalled in controller 300 or processor 302. Software functionalitiesmay involve programming, including executable code, and suchfunctionalities may be used to implement the methods disclosed herein.Such software code is executable by the general-purpose computer or bythe processor unit described below.

In operation, the code and possibly the associated data records arestored within a general-purpose computer platform, within the processorunit, or in local memory. At other times, however, the software may bestored at other locations and/or transported for loading into theappropriate general-purpose computer systems. Hence, the embodimentsdiscussed herein involve one or more software products in the form ofone or more modules of code carried by at least one machine-readablemedium. Execution of such code by a processor of the computer system orby the processor unit enables the platform to implement the catalogand/or software downloading functions, in essentially the mannerperformed in the embodiments discussed and illustrated herein.

The computer and/or processor as discussed below may each employ acomputer-readable medium or machine-readable medium, which refers to anymedium that participates in providing instructions to a processor forexecution, including for example, determining an amount of pressureassociated with a touch event, as explained below. Any memory discussedbelow constitutes a computer-readable medium. Such a medium may takemany forms, including but not limited to, non-volatile media, volatilemedia, and transmission media. Non-volatile media include, for example,optical or magnetic disks, such as any of the storage devices in anycomputer(s) operating as one of the server platforms, discussed above.Volatile media include dynamic memory, such as main memory of such acomputer platform. Physical transmission media include coaxial cables;copper wire and fiber optics, including the wires that comprise a buswithin a computer system.

Common forms of computer-readable media therefore include, for example:a floppy disk, a flexible disk, hard disk, magnetic tape, any othermagnetic medium, a CD-ROM, DVD, any other optical medium, less commonlyused media such as punch cards, paper tape, any other physical mediumwith patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, anyother memory chip or cartridge, a carrier wave transporting data orinstructions, cables or links transporting such a carrier wave, or anyother medium from which a computer can read programming code and/ordata. Many of these forms of computer readable media may be involved incarrying one or more sequences of one or more instructions to aprocessor for execution.

Light-Guiding Assembly

FIG. 3 is a cross-sectional view of example system 10 that includes anexample embodiment of assembly 20. Assembly 20 includes a top opticalwaveguide (“top waveguide”) 22A and a bottom optical waveguide (“bottomwaveguide”) 22B. Top and bottom waveguides 22A and 22B have respectiveinput edges 23A, 23B adjacent light source 100 and respective outputedges 24A and 24B adjacent detector 200. The top and bottom waveguides22A and 22B also have respective bodies 25A and 25B that aresubstantially transparent to the wavelength of light 104 emitted bylight source 100. Top and bottom waveguides 22A and 22B also haverespective upper surfaces 26A and 26B, and respective lower surfaces 27Aand 27B that are defined by respective bodies 28A and 28B.

In an example embodiment, the lower surface 27A of top waveguide 22A andthe upper surface 26A of lower waveguide 22B are spaced apart by a gap30. In one example, gap 30 is filled with a solid material while inanother example is filled with air. Solid materials may comprise lowindex, transparent, low modulus, and/or high elasticity materials thathave limited absorption of the evanescent wave. Gap 30 has a thicknessTH₃₀ that in an example can range from 0 microns (i.e., no gap) to about2 mm. In an example, gap 30 is formed by spacers 32. Other thicknessTH₃₀ for gap 30 are possible depending on the particular application forassembly 20.

In an example, top and bottom waveguides 22A and 22B are defined byfirst and second transparent and generally planar glass sheets. In otherembodiments, materials other than glass can be used, such as polymers,plastics and other non-glass materials that are substantiallytransparent at the wavelength of light 104.

System 100 includes an input optical system 123 operably arrangedbetween light source 100 and input edges 23A and 23B of top and bottomwaveguides 22A and 22B. System 100 also includes an output opticalsystem 124 operably arranged between light source 100 and output edges24A and 24B of top and bottom waveguides 22A and 22B. Input opticalsystem is configured to receive (coherent) light 104 from light source100 form therefrom collimated (coherent) light beams 104A and 104B, andto direct these light beams into top and bottom waveguides 22A and 22B,respectively. Light beams 104A and 104B travel through top and bottomwaveguides 22A and 22B over respective optical paths OPA and OPB tooutput edges 24A and 24B. Optical paths OPA and OPB are separate opticalpaths within top and bottom waveguides respectively, i.e., light beams104A and 104B do not overlap with each other as they travel throughtheir respective waveguides.

Output optical system 124 is configured to receive the light beams 104Aand 104B that exit respective output edges 24A and 24B and combine thebeams so that they interfere, thereby forming an interfered light beam104AB. The interfered light beam 104AB is directed to detector 200,which detects the interfered light beam. Generally, top and bottomwaveguides can have any reasonable configuration that allows them toguide respective light beams 104A and 104B.

In some embodiments, optical paths OPA and OPB represent thelowest-order mode of the top and bottom waveguides 24A and 24B. Thus,input and output optical systems 123 and 124 can also be referred to asmode conditioners, since they are used to excite and detect a selectmode (here, the lowest-order mode) of top and bottom waveguides 24A and24B. In some embodiments, the optical paths OPA and OPB represent highermodes or a combination of modes.

Top and bottom waveguides 22A and 22B may generally be made of anysuitably transparent material that can be formed into a thin planarsheet, such as plastic, acrylic, glass, etc., and that supports thetransmission of light beams 104A and 104B within their respective bodies28A and 28B without substantial loss due to scattering or absorption. Inan example embodiment, top and bottom waveguides 22A and 22B havethicknesses TH_(A) and TH_(B) such that the waveguides can flex withoutbreaking when pressure is locally applied at top surface 22 at touchlocation TL. An exemplary range for thickness THA is from 0.3 mm to 0.8mm, and for THB is from 0.3 to 1.5 mm. In some embodiments, THA is 0.3,0.4, 0.5, 0.6, 0.7, or 0.8 mm. In some embodiments, THB is 0.3, 0.4,0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5 mm. Otherthickness can be employed consistent with the particular application fortouch system 10. In an example, only top waveguide 22A has a thicknessthat allows it to flex, while bottom waveguide 22B is sufficiently thickor is otherwise rigid that it retains its generally planer configurationeven when the top waveguide is subject to a flexing force due to a touchevent TE.

In an example embodiment, one or both of top and bottom waveguides maybe formed from a chemically strengthened glass, such as a soda-lime-typeglass. An example glass for top and bottom waveguides 22A and 22B is analkali aluminosilicate glass hardened through ion exchange. These typesof glass can comprise Na₂O (soda), CaO (lime) and SiO₂ (silica), but canalso include oxides such as MgO, U₂O, K₂O, ZnO, and ZrO₂. Once hardenedthrough ion exchange, these types of glass exhibit certaincharacteristics that make them desirable for touch screen applications,as well as other applications (e.g., as a cover glass). Further detailsas to the formulation or production, or both, of soda-lime-type glasssuitable for use as waveguiding sheets may be found in one or more ofU.S. patent application Ser. No. 11/888,213 filed Jul. 31, 2007; U.S.patent application Ser. No. 12/537,393 filed Aug. 7, 2009; U.S. patentapplication Ser. No. 12/545,475 filed Aug. 21, 2009; and U.S. patentapplication Ser. No. 12/392,577 filed Feb. 25, 2009. An exemplary glassfor use herein is Gorilla® glass, from Corning, Incorporated, Corning,N.Y. Also, an exemplary glass, such as low-iron Gorilla® glass or otherlow-iron ion-exchanged glass, is transparent to IR-wavelength light 104.

The configuration of assembly 20 defines an interferometer wherein topand bottom waveguides 22A and 22B define the two different arms of theinterferometer over which light 104A and 104B respectively travel. This,in an example embodiment, interfered light 104AB is formed from light104A and light 104B that travel through separate (i.e., non-overlapping)optical paths over the two arms of the interferometer. In an example,the interferometer is configured to be unbalanced so that interferenceoccurs at detector 200. This can be accomplished by making optical pathsOPA and OPB different from each other, e.g., by having one of top andbottom waveguides 22A and 22B slightly longer than the other. Thisintroduces an initial or first optical path difference ΔOPD₁.

When there is no touch event TE occurring on upper surface 26A of topwaveguide 22A, optical paths OPA and OPB remain constant so that thefirst or initial optical path difference ΔOPD₁ stays the same. When atouch event TE occurs on upper surface 26A of top waveguide 22A, itcreates a change in the at least one of optical paths OPA and OPB givingrise to a second optical path difference ΔOPD₂ that causes a change inthe interfered light 104AB detected by detector 200.

FIG. 4A is a cross-sectional view of top and bottom waveguides 22A and22B when upper surface 26A is subject to a touch event TE at a touchevent location TL that bends the top waveguide. It is assumed that boththe top and bottom waveguides have a length L when no touch event TE isoccurring. When a touch event TE occurs at touch location IL, topwaveguide 22A bends with a radius of curvature R measured relative to acentral longitudinal axis of the top waveguide. This is just one bendingscenario and other bending scenarios are discussed below.

The new optical path OPA′ associated with the flexed top waveguide 22Ais longer than the optical path OPA of the unflexed top waveguide. Theoptical path OPB of bottom waveguide 22B remains substantiallyunchanged. This is true whether the bending is over the entire length Lof top waveguide 22A or whether the bending is more localized.

Assuming that the curvature of top waveguide 22A has a radius ofcurvature R=r+δr, and that curvature subtends an angle θ, then if θ is arelatively small angle (i.e., R is relatively large, which is areasonable assumption in the present instance), it can be readily shownthat change in the optical path from OPA to OPA′ is approximately δr.Thus, the optical path difference between the optical paths OPA′ and OPBof the top and bottom waveguides 22A and 22B, which can also be denotedas ΔOPD₂, is about δr. If δr is for example about 2.5 microns, thatwould be roughly equal to two wavelengths of infrared light ofwavelength of 800 microns or so. This amount of optical path differenceΔOPD₂, would manifest itself as a change in the interfered light 104AB,e.g., a change in the amount of optical power due to shifting fringes inthe detected portion of the resulting interference pattern.

FIG. 4B is similar to FIG. 4A and illustrates an example of assembly 20where top and bottom waveguides 22A and 22B are separated by a very thinlayer 33 of material so that both waveguides flex when subjected topressure at touch location TL. Top waveguide 22A flexes to have a radiusof curvature R_(A) while bottom waveguide 22B flexes to have a radius ofcurvature R_(B). It can be shown that the optical path difference ΔOPD₂in the optical path lengths OPA′ and OPB′ associated with the flexedwaveguides is approximated by θ(R_(B)-R_(A)). Again, this is true in thecase when the bending of the two waveguides is localized.

For θ=1°, R=1 m, and if the center-to-center separation of the guides isR_(B)-R_(A)=0.5 mm, ΔOPD₂=8.3 microns and the maximum deflection δr=0.15mm at touch location TL. The nominal length L of the top and bottomwaveguides 22A and 22B from their respective input ends 23A, 23B totheir respective output ends 24A, 24B is about 17 mm. The values ofthese parameters are all consistent with being able tointerferometrically detect the occurrence of a touch event TE.

FIG. 5A is an elevated view of an example assembly 20 showing topwaveguide 22A, bottom waveguide 22B with layer 33 in between. Layer 33may be transparent or opaque, or may not be included in assembly 20.Layer 33 may be relatively thin or may be sufficiently thick toconstitute a support layer or substrate. In an example, top waveguide22A is made of a first material having a modulus of elasticity E1 whilebottom waveguide 22B is made of a material having a modulus ofelasticity E2, wherein E2<E1. This configuration limits the flexing ofthe combined structure to the location where the force or pressure isapplied. Said differently, the radius of curvature of the bending isstronger and more localized.

FIG. 5B is similar to FIG. 5A and illustrates an embodiment of assembly20 where top and bottom waveguide 22A and 22B each have a modulus ofelasticity E1 and are supported on a support substrate 86 having anelastic modulus E2, wherein E2<E1. As with the configuration of FIG. 9A,the configuration of FIG. 8B also limits the flexing of the combinedstructure to the location where the force or pressure is applied, i.e.,the radius of curvature of the bending is stronger and more localized.In an example, top and bottom waveguides 22A and 22B can have a modulusof elasticity E1A and E1B, and the condition E2<E1B<E1A is satisfied.

FIGS. 5C and 5D are cross-sectional views of an example assembly 20wherein top waveguide 22A is sufficiently thin and the underlyingsupport substrate 80 or 86 (i.e., a transparent or an opaque supportsubstrate) is sufficiently flexible so that the touch event TE causes amore localized curvature at the touch event location TL. In FIGS. 5C and5D, finger 50 is shown as causing touch event TE. Note that the radii ofcurvature R_(A) and R_(B) are smaller than in the case where the bendingof the top waveguide (or both of the top and bottom waveguides) occursover a larger distance.

FIG. 6A is a cross-sectional view of an example embodiment of assembly20 wherein top and bottom waveguides 22A and 22B are formed in the body84 of a transparent sheet 80 and have a gradient index. In an example,the gradient-index top and bottom waveguides are formed using anion-exchange process. The light beams 104A and 104B that respectivelytravel in top and bottom waveguides 22A and 22B are shown as afundamental guided mode in each of the gradient-index waveguides.

FIG. 6B is similar to FIG. 6A and illustrates an example embodimentwherein the top and bottom waveguides 22A and 22B are formed as layeredstructures on the top and bottom sides of support substrate 86 that hasa body 88 that need not be transparent to the operating wavelength oflight. Top and bottom waveguides 22A and 22B are shown as being formedby a high-index core layer 90 that includes a lower-index cladding layer92 adjacent the core layer. The bulk index of body 88 serves as thecladding layer on the other side of core layer 90. In an example,support substrate 86 is the same as transparent glass sheet 80. Inanother example, support substrate 86 is made of a flexible materialsuch as plastic, acrylic, thin metal, etc. If body 88 is opaque, thenthe top and bottom waveguides 22A and 22B each require an additionalcladding layer 94 adjacent body 88 of support substrate 86, asillustrated in FIG. 6C.

Touch System Operation

In the general operation of touch system 10, controller 300 viaprocessor 302 drives the activation of light-source 100 via alight-source signal SL and also controls the detection of interferedlight 104AB at detector 200. Detector 200 generates detector signal SDin response to detecting interfered light 104AB, wherein the strength ofthe detector signal SD is representative of the intensity of thedetected interfered light. Portions of interface circuit 306 can beplaced near detector 200. For example, preamplifiers andanalog-to-digital converters (not shown) may be placed near detector 200to eliminate noise that may be induced in long wires between processor302 and the detectors 200, particularly when the processor is centrallylocated.

In an example, processor 302 controls the light emission and detectionprocess to optimize the detection of interfered light 104AB, e.g., byproviding a characteristic (e.g., a modulation) to the light 104 fromthe light-source elements 102, or by gating detectors 200 to reducenoise, etc., or both. The modulation may be wavelength modulation orintensity modulation.

FIGS. 7A through 7C illustrate an example of how a finger 50 is used tocreate a touch event TE at a touch location TL, with the touch eventcausing enough pressure to flex top and bottom waveguides 22A and 22B.In the example embodiments shown in FIGS. 7A through 7C, assembly 20includes a frame 60 at perimeter 27, wherein the frame supports top andbottom waveguides 22A and 22B as well as layer 33 sandwiched between.The flexing of assembly 20 is shown as being gradual. In otherembodiment such as those described above in connection with FIGS. 5C and5D, the bending can be more localized. In an example, the bending can bea combination of localized bending or deformation and a more gradual,larger-scale flexing.

FIG. 8A is a cross-sectional diagram of an example assembly 20 thatillustrates an example configuration for input and output opticalsystems 123 and 124. In the example embodiment shown, input and outputoptical systems include respective lenses 133 and 134, which can eachcomprise one or more lenses, lens elements, or other optical elements.Lens 133 is configured to receive diverging light 104 from light source100 and form collimated light 104A and 104B. Likewise, lens 134 isdesigned to receive collimated light beams 104A and 104B and focus themto form interfered light 104AB at detector 200. The combination of lightsource 100 and input optical system 123 defines a light source system103, while the combination of detector 200 and output optical system 124defines a detector system 203 (see FIG. 3).

FIG. 8B is similar to FIG. 8A and illustrates an example embodimentwhere optical fibers are used to configure the input and output opticalsystems 123 and 124. Input optical system 123 includes a first opticalfiber section F1 that is optically connected at one end to light source100 and at the other end to a fiber optic coupler 140. Fiber opticcoupler 140 is also optically coupled to respective ends of two otheroptical fiber sections F2A and F2B. First and second lenses 133A and133B are arranged adjacent output ends of optical fiber sections F2A andF2B, respectively. Lenses 133A and 133B may each comprise one or morelenses, lens elements, or other optical elements. Light 104 from lightsource 100 is carried to coupler 140 by fiber section F1. Coupler 140serves to split the light between optical fiber sections F2A and F2B sothat light beams 104A and 104B respectively travel in these fibers.Lenses 133A and 133B are arranged adjacent the output ends of opticalfiber sections F2A and F2B and collimate light 104A and 104B and directthe collimated light into respective top and bottom waveguides 22A and22B.

At the detector end, output optical system is configured in essentiallythe same manner but in reverse. Thus, collimated light beams 104A and104B from top and bottom waveguides 22A and 22B are received byrespective lenses 134A and 134B that focus these light beams intorespective optical fiber sections F2A and F2B on the detector side.Lenses 134A and 134B may each comprise one or more lenses, lenselements, or other optical elements. Light 104A and 104B travels inthese fiber sections to coupler 140, where they are combined to forminterfered light beam 104AB. Interfered light beam 104AB then travels infiber section F1 from coupler 140 to detector 200, which then detectsthe interfered light beam.

Detector 200 is configured to convert the detected interfered light beam104AB to an electrical signal embodied by detector signal SD, which inan example is a photocurrent. The detector may be configured to measuresignal via a variety of methods, including overall signal intensity,integrated signal intensity, signal intensity at a particularwavelength, signal frequency, changes in signal frequency, etc. Thedetector electrical signal SD is then sent to processor 302 forprocessing, as described below. Such processing can be used to extractinformation relating to changes in the applied pressure at top surface26A of top waveguide 22A created by touch event TE at touch location TL.

FIG. 8C is similar to FIG. 8B, except that bottom waveguide 22B is inthe form of optical fiber section F2B, which now directly connects thetwo fiber couplers 140 and serves as the second arm of theinterferometer.

Aspects of the disclosure include determining the time-evolution of thetouch event TE. This can be used for example to determine a relativeamount of pressure that is applied to upper surface 26A of top waveguide22A by, for example, a finger, a stylus, the eraser-end of a pencil, orlike implement. In the discussion below, a finger and a stylus with acompressible end are used by way of non-limiting example to describe thepressure-sensing capabilities of touch system 10. Aspects of thedisclosure include detecting respective pressures associated multipletouch events, such as when touch system 10 is used in forming apressure-sensing keyboard, as discussed below. It is noted here that thepressure applied to assembly 30 at top waveguide 22A may be throughanother surface that resides atop surface 26A, such as a coating layer.

FIGS. 9A and 9B are plots of wavelength λ vs. detector optical power(“power”) in normalized units in the case where the optical pathdifference ΔOPD₂ between the top and bottom waveguides is 7.5 μm and 15μm, respectively. It can be seen from FIGS. 9A and 9B that the detectorsignal SD from detector 200 can vary as a sine wave that changesfrequency as the optical path difference ΔOPD₂ increases. Thus, bymonitoring the change in frequency of detector signals SD, one candetermine the time-evolution of the deflection in assembly 20 due to atouch event. This time-evolution of the touch event can be used toassess the amount of pressure vs. time being applied to the touchlocation TL. It can also be used to sense multiple touches insuccession, i.e., a double-tap.

In an example embodiment, light source 100 is wavelength modulated viaprocessor 302 of controller 300 via light source control signal SL (seeFIG. 1). This causes a change in interfered light beam 104AB by changingthe optical paths OPA and OPB of light beams 104A and 104B in a selectmanner. This select change manifests itself in detector signal SD, whichis processed (filtered) by controller 300 to bring out the lasermodulation frequency. This can be accomplished, for example, by alock-in amplifier.

Thus, in an example embodiment, the time evolution of the processeddetector signal SD is used to characterize the time evolution of touchevent TE. For example, as an object (e.g., finger 50, stylus, etc.)initially contacts surface 26A of top waveguide 22A, a small amount offlexing of the transparent sheet occurs. As finger F continues to pushinto top waveguide 22A, the amount of flexing of the top waveguidechanges, so that optical path length difference OPD₂ continuouslychanges. The changing optical path lengths show up as a continuouschange in the processed detector signal SD.

Once the object applies a constant pressure at touch location TL, theprocessed detector signal SD stops changing. At this point, the amountof time that the processed detector signal SD remains constant can bemeasured. This information can be used, for example, to carry out afunction by requiring the touch event to have associated therewith notonly a certain amount of pressure but a select duration as well. Furtherin the example embodiment, it can be required that the touch event havea select time evolution in pressure that is consistent with say a fingeror stylus used to intentionally cause a touch event, as opposed to sayan arbitrary object pressing down on the surface 26A of top waveguide 2Aand inadvertently triggering a touch event.

FIGS. 10A and 10B are top-down views of system 10, wherein FIG. 10Ashows an example where light beams 104A and 104B travel over respectiveoptical paths OPA and OPB from light source 100 to detector 200 in topand bottom waveguides (not shown) of assembly 20. Only a single lightsource element 102 and detector element 202 are shown in FIG. 10A, whilean extended light source and an extended detector 200 are shown in FIG.10B.

FIG. 10B shows a keyboard 70 that includes indicia denoting for examplethe usual typing keyboard keys 72. The close-up inset view of key “F”shows that the key area is optionally provided with surface roughnessdenoted 29 that increases the amount of light scattering for the keys,particularly when they are pressed down upon with a finger to create atouch event. This may be used to illuminate keys 72 using guided light104A. While guided light 104 travels straight down top waveguide 22A,the light therein interacts with upper surface 26A. If this surface issmooth, then there is very little if any loss from scattering. Theintroduction of surface roughness 29 will allow guided light 104A tointeract with select portions of upper surface 26A and allow some ofthis light to escape and be visible to a viewer or user of the keyboard.

Fiber-Based Assembly

FIG. 11A is a cross-sectional view of an example dual-core optical fiber320 that includes two optical fiber cores 322A and 322B embedded in acladding 323. FIG. 11B is a cross-sectional view of a fiber assembly 330that has two optical fibers 320A and 320B disposed adjacent one anotherand that include respective cores 322A and 322B surrounded by respectivecladdings 323A and 323B. In an example, cores 322A and 322B are formedsuch that they support just a single mode at the operating wavelength ofthe dual-core optical fiber.

Optical fibers such as dual-core fiber (“fiber”) 320, fiber assembly 300or like optical fiber configurations can be used to form a Mach-Zehnderinterferometer. Examples of such interferometers are disclosed in U.S.Pat. No. 5,295,205, U.S. Pat. No. 5,943,458, U.S. Pat. No. 5,351,325 andU.S. Pat. No. 6,862,396. FIG. 12 is taken from FIG. 1 of the '458 patentand illustrates an example fiber-based interferometer that includes apair of fibers F₁ and F₂. The fibers are coupled to one another forlight transfer therebetween at a first coupler C₁ and a second coupleC₂. The couplers are arranged to transfer light, one fiber to the other.Couplers C₁ and C₂ may be so-called “evanescent” couplers in whichnarrowed, elongated portions of the fibers are closely juxtaposed withone another within a matrix or outer coating. The couplers may be 3 dBcouplers, arranged to transfer approximately one-half of the opticalpower supplied on one fiber to the other fiber.

Fibers F₁ and F₂ have phase-shift regions with different optical pathlengths disposed between the couplers. Thus, the optical path lengthover the phase shift region in fiber F₁ is different from the opticalpath length over the phase shift region in fiber F₂. The optical pathlength difference has been provided either by making the phase shiftregion of one fiber physically longer than the other, or by making thetwo fibers F₁ and F₂ with different propagation constants so that thevelocity of light within the two fibers is different. The latterstructure can be achieved by making the fibers with different refractiveindex profiles. Where the fibers are “step-index” fibers, incorporatinga core having a relatively high refractive index and a coating with arelatively low refractive index overlying the core, the two fibers mayhave cores of different refractive indices, different core diameters,different coating refractive indices or some combination of these.

Regardless of the particular mechanism used to produce the optical pathlength difference, the single stage Mach-Zehnder filter depicted in FIG.12 will direct light supplied through input 1 either to output 3 or tooutput 4 depending upon the wavelength of the light. More complexMach-Zehnder devices utilize multiple stages with multiple phase shiftregions and multiple couplers connected in series to achieve certaindesirable wavelength-selective characteristics. Still other Mach-Zehnderdevices incorporate more than two fibers connected in parallel betweenthe couplers, as described in the aforementioned U.S. Pat. No.5,351,325. Various optical fibers incorporate different optical pathlengths. Desirably, the optical path length differences are selected toprovide optical path length differences that are rational or integralmultiples of one another. In FIG. 12, “OPLD” can also represent theoptical path length difference that arises due to a touch event TE, asexplained below.

For Mach-Zehnder devices to provide touch-sensing capability, the pathlength differences should be as specified and should remain stableexcept when deliberately altered, such as by a touch event TE. It isnoted that in an example embodiment, a light source element 102 (andinput optical system 123) is optically coupled into one of input 1 or 2,and detector element 202 (and output optical system 124) is provided atone of output 3 and 4. Alternatively, first and second detector elements202 and corresponding output optical systems 124 can be placed atoutputs 3 and 4 respectively to better detect interfered light 104AB.For example, depending on the phase difference between light beams 104Aand 104B, the amount of interfered light 104AB being outputted at ends 3and 4 can vary, so it can be more accurate to measure the amount ofpower in both of these output ends to ensure an accurate measurement ofthe optical path difference ΔOPD₂ caused by a touch event TE.

FIG. 13A is an elevated view of an example assembly 20 that includesupper and lower arrays 350U and 350L of optical fibers 320 that form anoptical fiber detection grid, as shown in the close-up view. This gridconfiguration can be used for (x,y) detection of the touch location TLof touch event since at a given touch location, the optical pathsdifferences between cores 322A and 322B will change in relation to theirproximity to the touch location. By detecting interfered light 104ABtraveling in each dual-core fiber, the (x,y) characteristics of theinterference effects can be determined by processing the correspondingdetector signals SD.

FIG. 13B is a close-up cross-sectional view of the example assembly 20of FIG. 13A. In an example, upper and lower fiber arrays 350U and 350Lare supported in a layer 354 of support material of relatively lowelastic modulus such as polyimide, which has an elastic modulus about 20times less than that of glass. Assembly 20 also includes a substrate 360that supports the support material 354 and the upper and lower fiberarrays 350U and 350L.

FIG. 14A is a cross-sectional view of one of fibers 320 in upper fiberarray 322A, and showing finger 50 just about the fiber. FIG. 14B issimilar to FIG. 14A, but shows finger 50 pressing down on fiber 320 toform touch event TE at touch location TL. Because fiber 320 is supportedin a manner that allows it to flex as part of the optical fiber gridformed by top and bottom fiber arrays 350T and 350L. Because of thebending of fiber 320, the length of optical paths OPA and OPB for light104A and 104B traveling in cores 322A and 322B change by differentamounts, thereby creating an optical path difference ΔOPD₂ in the samemanner as the planar versions of top and bottom waveguides 22A and 22B.

FIG. 14C is similar to FIG. 14B and illustrates an example embodimentwhere the bending of fiber 320 is more localized at the touch locationTL based on the construction of the fiber. For example, fiber 320 may bemade of more flexible materials than a conventional dual-core opticalfiber.

Display System

Touch system 10 can be used in combination with conventionalposition-sensing display systems, such as those that arecapacitive-based and resistive-based.

FIG. 15A is a schematic elevated view of an example pressure-sensingdisplay 400 formed by operably arranging touch system 10 adjacent andabove (e.g., atop) a conventional display unit 410, such as a liquidcrystal display, which display may have conventional position-basedsensing capability.

FIG. 15B is a schematic cross-sectional, partial exploded view of anexample touch-sensitive display 400 illustrating an example of how tointegrate touch system 10 with conventional display unit 410. Theconventional display unit 410 is shown in the form of a liquid crystaldisplay that includes a backlighting unit 414 that emits light 416, athin-film transistor (TFT) glass layer 420, a liquid crystal layer 430,a color filter glass layer 450 with a top surface 452, and a toppolarizer layer 460 with a top surface 462, all arranged as shown. Aframe 470 is disposed around the edge of color filter glass layer 450.Light source 100 is shown by way of example as being operably supportedwithin frame 470. This forms an integrated display assembly 480 having atop side 482.

To form the final touch-sensitive display 400, assembly 20 is added tointegrated display assembly 480 of conventional display unit 410 byoperably disposing the assembly on top side 482. The assembly 20 caninclude the aforementioned cover 40 in the form of an IR-transparent butvisibly opaque layer disposed adjacent light source 100 and detector200.

In an example, various indicia or indicium (not shown) such as keyboard70 (see FIG. 8B) may be presented to user 500 on or through top surface26A to guide the user to interact with touch system 10. By way ofexample, the indicium may include areas on top surface 22 of transparentsheet 20 that are set aside for indicating user choices, softwareexecution, etc., or to indicate a region where the user should createtouch event TE. Such region, for example, might be required where light104 does not reach certain portions of top surface 26A.

Although the embodiments herein have been described with reference toparticular aspects and features, it is to be understood that theseembodiments are merely illustrative of desired principles andapplications. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the appended claims.

What is claimed is:
 1. A touch system for use atop a display unit andfor sensing a localized touch event, comprising: first and secondwaveguides that define first and second separate optical paths havingrespective first and second optical path lengths that respectivelychange by first and second amounts by the localized touch eventoccurring directly on an upper surface of the first waveguide andcausing a first amount of deformation in the first waveguide and asecond amount of deformation in the second waveguide that is differentfrom the first amount of deformation; at least one light source systemoperably arranged relative to respective input ends of the first andsecond waveguides and configured to input first and second light beamsinto the first and second waveguides, respectively; and at least onedetector system operably arranged relative to respective output ends ofthe first and second waveguides and configured to interfere and detectthe first and second light beams and generate a detector signalrepresentative of the amount of deformation in the first and secondwaveguides caused by the localized touch event.
 2. The system accordingto claim 1, further comprising a controller operably coupled to the atleast one light source and the at least one detector and configured toreceive the detector signal and determine a change in an amount ofpressure applied at the localized touch event location based on theamount of deformation.
 3. The system according to claim 2, furthercomprising the light source being wavelength modulated to form intensitymodulated light at the detector.
 4. The system according to claim 1,wherein the first waveguide is defined by a first transparent, planarglass sheet having said upper surface.
 5. The system according to claim4, wherein the position of the localized touch event on the uppersurface of the planar glass sheet may be determined by the touch system.6. The system according to claim 5, wherein an amount of pressureapplied at the touch event location may be determined based on theamount of deformation.
 7. The system according to claim 1, wherein thefirst and second waveguides are transparent to infrared (IR) light,wherein the emitted light from the at least one light sources comprisesIR light, and wherein the at least one detector is configured to detectthe IR light.
 8. The system of claim 1, wherein the first and secondwaveguides are defined by first and second transparent, planar glasssheets.
 9. The system of claim 1, wherein the light source systemcomprises at least one light source and at least one input opticalsystem operably arranged between the light source and the input ends ofthe first and second waveguides.
 10. The system of claim 9, wherein theinput optical system is configured to receive diverging light from thelight source and form collimated light that defines the first and secondlight beams.
 11. The system of claim 10, wherein the input opticalsystem includes an input optical fiber section that is joined to firstand second optical fiber sections via an optical fiber coupler.
 12. Thesystem of claim 1, wherein the detector source system comprises at leastone detector and at least one output optical system operably arrangedbetween the detector and the respective output ends of the first andsecond waveguides.
 13. The system of claim 12, wherein the outputoptical system is configured to receive collimated first and secondlight beams from the respective output ends of the first and secondwaveguides and combine the first and second light beams to form aninterfered light beam.
 14. The system of claim 13, wherein the inputoptical system includes an output optical fiber section that is joinedto first and second optical fiber sections via an optical fiber coupler.15. A display system that has pressure-sensing capability, comprising:the touch system according to claim 1; and the display unit having adisplay, with the touch system operably arranged atop the display. 16.The display system of claim 15, wherein the display includes one ofcapacitive and resistive touch-sensing capability.
 17. A method ofsensing a localized touch event on a touch system arranged atop adisplay unit and that includes first and second waveguides, comprising:sending first and second light beams through input ends of the first andsecond waveguides that respectively have first and second separateoptical paths that define an optical path difference, with the first andsecond waveguides configured so that the localized touch event occursdirectly on an upper surface of the first waveguide and causes a changein the optical path difference by deforming both of the first and secondwaveguides by different amounts; combining the first and second lightbeams at output ends of the first and second waveguides to forminterfered light; and detecting the interfered light to generate adetector signal representative of the change in the optical pathdifference.
 18. The method of claim 17, further comprising determiningfrom the detector signal the amount of pressure applied at a touchlocation of the touch event.
 19. The method of claim 17, furthercomprising processing the detector signal using a computer having aprocessor configured to perform signal processing.